1. Technical Field of the Invention
The present invention relates to display systems, and, in particular, to a method and apparatus for providing non-genlocked live video on a computer system.
2. Description of Related Art
Personal computer (PC) and television (TV) technologies are presently converging. One of the products of this convergence is a single integrated device for information and entertainment, which device can, at least in part, utilize the available communications bandwidth, mass storage and graphics handling capabilities of the PC to deliver, store and display applications during a traditional TV viewing environment.
In spite of many recent advances in this area, several problems persist. One of the more nettlesome difficulties relates to the presentation of live video on the display monitor associated with a PC. It is well-known in the art that the frame rate of the incoming video must be synchronized to the frame rate of the graphics display system associated with the PC in order to present high quality video thereon. If there is no synchronization, a video anomaly known as "tearing" occurs. Tearing, which is generally obvious and unsettling to viewers, is caused when an update to graphics memory is performed across a graphics controller refresh pointer. As is well known, the severity of the effects of tearing are proportional to the amount of graphics memory that is changed, the location of the graphics controller refresh pointer when the change occurs, and the difference between the old and new graphics memory data. In practical terms, tearing can be caused by writing to the graphics memory (for example, via a bit logical transfer, or "BLT," or when live video data streams in for display) or by changing the memory pointer that the graphics controller uses to refresh the display in the middle of the graphics display sweep.
It is clear that to provide highest quality video on a PC, the graphics display must be genlocked, or synchronized, to the video source. If genlock is possible, a double-buffer memory structure is sufficient to prevent tearing even if the video is displayed at less than full graphics screen resolution. However, as can be appreciated by those skilled in the art, it may not always be possible to genlock the graphics display to the video source. There are several possible reasons for this. For example, it may be because the graphics subsystem hardware in the PC does not support it--currently, few graphics subsystems do. It may also be because more than one source is in use (as would be the case if "picture-in-picture," i.e. "PIP," is in use); it is well-known that it is not feasible to genlock the graphics display to more than one source.
Furthermore, it is well known that if the graphics refresh is not genlocked to the video source, it may be necessary to either drop a video frame or repeat the display of a video frame in order to maintain display synchronization. However, dropping video frames at irregular intervals can result in what are sometimes known as instantaneous syncopation artifacts, highly undesirable visual effects that are akin to the effect caused when a video is played back at a frame rate that is not a whole-number multiple of the frame rate at which the video was recorded. For example, when a film that was recorded at 24 frames per second is transferred to video at 60 fields per second a "three-two pull-down" is performed. The resulting video is composed of three fields from one film frame and two fields of the next film frame. When the video is displayed, one film frame is displayed for 1/2.sup.th of a second and the next film frame is displayed for 1/30.sup.th of a second. This syncopation effect can result in visibly "jerky" motion, especially for smoothly scrolling objects. As is well known in the art, dropping frames at irregular intervals may create the most severe and visible instantaneous syncopation effects. In general, the severity of syncopation effects varies with the frequency and magnitude of the mismatches between the recorded and playback frame rates.
Accordingly, based upon the foregoing, it should be understood and appreciated that there is a need for a display system that can display non-genlocked live video on a monitor without the aforementioned anomalies. Although two-buffer display systems have been extant for sometime, no such system is known to have all of the advantages and novel features of the system described and claimed hereinbelow.